Adhesive tapes and labels having a laminate construction are well known. In a typical construction, one or more layers of adhesive are coated on or otherwise applied to a release liner, and then laminated to a facestock, such as paper, polymeric film, or other flexible material. The adhesive may be a pressure-sensitive adhesive (PSA), and may be rubber-based or acrylic-based. Rubber-based hot melt PSAs (HMPSAs) typically contain one or more natural or synthetic elastomers, tackified with a petroleum resin, rosin or rosin derivative, and/or other ingredients, such as plasticizers, which improve the tack of the adhesive.
During label manufacture, a laminate of a facestock, PSA layer and a release liner is passed through apparatus that converts the laminate into commercially useful labels and label stock. The processes involved in the converting operation include printing, slitting, die-cutting and matrix-stripping to leave labels on a release liner, butt-cutting of labels to the release liner marginal hole punching, perforating, fan folding, guillotining and the like.
Die-cutting involves cutting of the laminate to the surface of the release liner. Hole punching, perforating and guillotining involve cutting clean through the label laminate.
The cost of converting a laminate into a finished product is a function of the speed and efficiency at which the various processing operations occur. While the nature of all layers of the laminate can impact the cost of convertibility, the adhesive layer typically has been the greatest limiting factor in ease of convertibility. This is due to the viscoelastic nature of the adhesive, which hampers precise and clean penetration of a die in die-cutting operations and promotes adherence to die-cutting blades and the like in cutting operations. Stringiness of the adhesive also impacts matrix-stripping operations, which follow die-cutting operations.
Achieving good convertibility does not, by necessity, coincide with achieving excellent adhesive performance. Adhesives must be formulated to fit specific performance requirements, including sufficient shear, peel adhesion, and tack or quick stick, at various temperatures. A good, general purpose adhesive may exhibit poor convertibility simply because the adhesive is difficult to cleanly cut. Such an adhesive may stick to a die or cutting blade. An ideal HMPSA would have both high tack and high cohesive strength, good flow characteristics if it is to be used in the bulk state, so that it can be coated or otherwise applied to a facestock, or coated on a release liner and laminated to a facestock, and good converting performance.
In an effort to address the sometimes conflicting demands of good adhesive performance and good convertibility, multilayer constructions having two or more layers of adhesives have been proposed and described.
U.S. Pat. No. 4,260,659 to Gobran, incorporated herein by reference, describes a multilayer PSA tape formed of a plurality of superimposed PSA layers, the outer layer of which is softer than the immediate underlying layer.
U.S. Pat. No. 4,894,259 to Kuller, incorporated herein by reference, describes a process for producing a PSA tape comprised of a plurality of concurrently coated layers, at least the outer layer of which is a PSA layer, with contiguous layers defining an interphase between each layer, the interphase comprising a photopolymerized matrix of polymer chains extending from the matrix of one layer to the interface to the matrix of a contiguous layer. When photopolymerized, the layers cannot be delaminated.
WO96/08367 to Ercillo et al., incorporated herein by reference, describes a multilayer PSA construction in which one layer is formed with a first adhesive having a first glass transition temperature and a second layer having a second glass transition temperature. The first glass transition temperature differs from the second glass transition temperature. To the extent a tackifier and/or plasticizer is incorporated in such multilayer constructions there is no specific instruction on how to control diffusion or migration of tackifiers and/or plasticizers.
WO96/08369 to Ugolick et al, incorporated herein by reference, describes a multilayer PSA construction in which a first layer is a barrier layer and a second layer is an adhesive layer on the barrier layer. The barrier layer is a pressure-sensitive adhesive and inhibits the migration of mobile species such as oils, resins, and tackifiers from the adhesive layer into the facestock. In one embodiment, the adhesive layer is an apolar rubber-based composition, and the barrier layer is a thermoplastic acrylic PSA. The construction is useful as a label, and is designed to improve adhesion and preserve the appearance of the label by inhibiting wrinkling and/or staining.
U.S. Pat. No. 5,290,842 to Sasaki et al, incorporated herein by reference, discloses a PSA construction that utilizes a mutually immiscible first elastomer--typically a styrene-butadiene (SB) and/or styrene butadiene-styrene (SBS) block copolymer--exhibiting a first glass transition temperature, and second elastomer--typically a styrene-isoprene (SI) and/or styrene-isoprene-styrene (SIS) block copolymer--exhibiting a second glass transition temperature greater than the first. The composition is tackified by a tackifying system comprising a tackifier that is preferentially miscible in the second elastomer to provide a composition exhibiting excellent processibility and ambient and low temperature properties.
One problem with multilayer adhesive constructions is diffusion and migration of tackifiers and plasticizers, especially those of low molecular weight, which causes an undesirable change of adhesive properties. Although an intermediate barrier layer can be positioned between adhesive layers, such an approach complicates the manufacturing process and increases the cost of the multilayer construction. One proposal for reducing tackifier migration between layers has been to provide substantially equal levels of liquid tackifiers and/or plasticizer oil additives to the two layers to prevent diffusion or migration of such materials from one layer to another. However, maintaining the same ratio of tackifiers and plasticizers to elastomers in two adhesive layers has not led to a completely satisfactory result.
A need clearly exists for a more effective means of preventing migration of liquid plasticizers and tackifiers between layers of multilayer adhesive constructions so as to retain the properties of the individual adhesive layers for extended periods of time. There is also a need to prevent or reduce tackifier migration into polyolefin facestocks. When a hot melt PSA formulated with rubber-based block copolymers is used with a polyolefin facestock, swelling and hardening of the facestock can occur over time. This manifests itself as a wrinkling of the adhesive construction and/or a loss of adhesion, and is believed to be caused by the migration of tackifiers and plasticizers utilized in the rubber-based adhesives into the facestock.